Ekofisk Oil Field - Subsidence

Subsidence

In the mid-1980s the Ekofisk field as a whole and the platforms in particular were found to be suffering from an unexpected degree of subsidence. Detailed geological investigation showed that it was the result of delayed compactional diagenesis of the Chalk Formation reservoir rocks. As hydrocarbons were produced and replaced with water, the Chalk began to re-dissolve due to high-pressure inter-grain contacts and redeposit in a more compact, lower porosity configuration. Movements of several metres were recorded, and it was calculated that the total subsidence would almost be 6 metres (20 ft) at the end of the concession of Phillips Petroleum, too much to keep the platforms secure.
The Norwegian government pressed Phillips to take action and the French company TECHNIP was ordered to find a solution. As 5 of 7 platforms were interconnected, they had to be jacked-up by about 6 metres (20 ft) at the same time. The solution was that the steel tubular legs of the platforms would be extended. Subsequently large flanges were welded to these legs and when all flanges were welded and the legs cut, five platforms would be lifted simultaneously in one operation then extension pipes would be mounted in between the flanges. After bolting all flanges the platforms would be safe again.

The four days lifting was completed on August 17, 1987 at 11:30 pm thanks to 108 hydraulic cylinders synchronised with a network of 14 NUM 760FCNCs. The position tolerance of the cylinders with each other (+/- 3 millimetres (0.12 in) for a 6 metres (20 ft) extension per platform and +/- 100 millimetres (3.9 in) between platforms) was to be kept for 38 hours. During the welding of the flanges to the legs, these hydraulic cylinders took over the entire load. A couple of days before this great jack-up, the hotel platform was lifted, as it was not interconnected with the others. The total lifting capacity of all these cylinders was approximately 40,000 tonnes (39,000 long tons; 44,000 short tons) and was published in the Guinness book of records as being the largest jack-up ever.